1. Field of the Invention
The present invention relates to the field of electrical machines and to a rotating electrical machine.
2. Brief Description of the Related Art
Laminated press plates have long been used in large turbogenerators with H2/H2O cooling (see, for example, “Erfolgreiche Nachrüstung eines 970 MVA Turbogenerators” [Successful modification of a 970 MVA turbogenerator], ABB Review 3/1988, pages 3 et seq.). The exemplary arrangement of such a press plate is reproduced in FIG. 1. FIG. 1 shows a detail of one end of a turbogenerator 10, which includes a central rotor 11 and a stator 12, which concentrically surrounds the rotor. The stator 12 has a laminated stator core 13, which is terminated at the two axial ends by laminated press plates 14. The press plates 14 are pressed against the laminated stator core 13 in the axial direction by means of clamping bolts 16 and press fingers 15, which are arranged distributed over the circumference. The function of the press plates 14 is, firstly, the introduction of an axial force of pressure on the laminated stator core 13 both in the yoke region JB and in the tooth region ZB, which is located on the inner circumference of the laminated stator core 13 and accommodates the stator winding, of which the end-side stator end winding 17 can be seen in FIG. 1 (the rotor 11 has a corresponding rotor end winding 17a). Secondly, the press plate 14 is provided for the low-loss guidance of part of the front stator magnetic field (see the magnetic lines of force 18 in FIG. 1).
The above-described laminated press plate (14 in FIG. 1) includes individual press plate laminates of the same quality and the same laminate section as the normal stator laminates. Such press plate laminates 20, which have the form of a ring sector and have radial slots 22 or teeth 19 for accommodating the stator winding bars on the inner circumference and can be equipped with first holes 21, which form a bore 23 for the clamping bolts in the press plate, and second holes 25 for forming axial cooling channels, are illustrated in FIGS. 3 and 4. With the conically extended press plate, in order to achieve a cone angle δ, preferably in the region of 15-20°, the laminates are punched back or recently completely cut by a laser beam in layers (FIG. 3b) or blocks (press plate 14′, FIG. 4b; block step 24). As in the laminated core, the press plate laminates 20 are also in overlapping layers in the press plate 14 or 14′.
All the laminates of the laminated press plate 14, 14′ have the conventional laminate insulation used for the laminates of the laminated stator core. The laminates of the press plate, in a last step of the press plate production, are adhesively bonded, under pressure, to form a ring. Such adhesive bonding can take place, for example, in a furnace of a total-immersion impregnation unit (global VPI). However, the laminates can also be coated with a baked enamel (B-stage resin) and are adhesively bonded to one another under pressure and heat. The press plate thus produced is absolutely free of shorts beneath the laminate segments.
Usually, prestressed clamping bolts (16 in FIG. 1) are provided in the yoke center over the entire circumference, which clamping bolts are supported in the press plates and thus hold the laminated core such that it is permanently pressed between the press plates.
The magnetic leakage field of the stator and rotor end winding (17 and 17a, respectively, in FIG. 1) is drawn in by the magnetic press plate and in the process enters the press plate normal to the cone (lines of force 18 in FIG. 1). Under ideal conditions, the incoming magnetic flux (xx in FIG. 1) is transferred proportionally in each laminate and deflected outwards in the radial direction and in the circumferential direction in the laminate (arrows in FIG. 1). The magnetic flux collects and closes in the circumferential direction over a pole pitch. The radial component of the flux corresponds to a normal component of the flux increased by a factor of 1/sin δ. With a cone angle δ of, for example, 15°, the radial component is approximately four times as great. The incoming flux bends around in the circumferential direction and accumulates by further absorbing radial components. This results in magnetic saturation of the press plate in the yoke region.
At the same time, a considerable component of the air gap main field (yy in FIG. 1) is superimposed slightly deeper in the inner region of the press plate. Superimposition of the components stator end winding leakage field, rotor end winding leakage field, and main field results. This superimposition is dependent on the load and on the phase angle between the stator current and the voltage and is expressed in a rotation of the rotor (angular displacement). In a known manner, during under excited operation (stator current leads the voltage), the most severe excessive increase in the axial field on the press plate occurs. In the case of the laminated press plate, the field attempts to compensate for itself by changing to laminate layers of the press plate which lie axially deeper.
In contrast to the conventional solid press plate, the laminated press plate absorbs the incoming front flux and guides it in sequence over a pole pitch. The above described tendency towards compensating axial flux in the press plate does have disadvantages, however:                1. Since the butt joint is offset tangentially to a large extent from layer to layer of the laminates, the axial flux cannot flow in any other way than through the laminates.        2. The axial flux results in a high degree of eddy current reaction in the laminate.        
The eddy current reaction can result in high temperatures. The point with the maximum load is in the slot base region of the yoke. In order to avoid over-heating owing to the incoming front field, it is a worldwide established practice to provide the teeth with a slit, which is open towards the bore, over a certain axial depth of the laminated core, if necessary (for example FIG. 4 in DE-A-29 24 037).
In limit-rating generators, heating of the laminates in the slot base region may be so severe during under-excited operation that an arching pressure builds up (yoke cold on the outside, hot on the inside), which causes adjacent laminate segments to touch one another despite an installation gap (generally of a few tenths of a millimeter) (image of the damage from the stator end part in FIG. 2), since the forces of the arching pressure are so severe that the adhesive bonds rip open and/or flow under the severe shear forces. Owing to the touching contact, electrical shorts occur which further increase the losses. Laminates can repel one another locally and, as a result, cause axial shorts. The magnetic air gap field yy (FIG. 1) of the machine will also inject a fault current into the fault location thus produced. Self-driven propagation of the fault location (core burning) may result.